Conventionally, a lead storage battery has been used for starting an engine of a vehicle and for a back-up power source. Among these usages, a lead storage battery for starting an engine functions to supply electricity to various electric and electronic devices mounted on vehicles, in addition to a cell motor for starting an engine. After starting an engine, a lead storage battery is charged by an alternator. An output voltage and an output current of the alternator are set so that SOC (state of charge) of the lead storage battery is maintained to be 90 to 100%.
In recent years, a demand for an improvement of a fuel-efficiency of a vehicle is increasing, in view of environmental conservation. For such a demand, a vehicle carrying a stop-and-go-system and a regenerative-braking-system has been considered, for example. In the stop-and-go-system, an engine is stopped while the vehicle is idling, and in the regenerative-braking-system, a kinetic energy of a vehicle at the time of deceleration is converted to an electric energy, and the electric energy is stored.
In a vehicle carrying the stop-and-go-system, the lead storage battery is not charged when the vehicle is stopped in an idle stop mode. The lead storage battery sometimes supplies electric power to devices mounted on the vehicle while in such a state. Thus, in comparison with a conventional lead storage battery for starting engines, SOC of the lead storage battery inevitably becomes low. In a vehicle carrying the regenerative-braking-system, SOC of the lead storage battery has to be controlled to be lower, to about 50 to 90%, since electric energy is stored by the lead storage battery at the time of regeneration (deceleration).
In any of these systems, charge and discharge (hereinafter referred to as charge/discharge) are repeated frequently with a lower SOC range than ever. Further, based on an increase in a dark current accompanied with vehicle parts increasingly becoming electrically powered, a discharge of the lead storage battery advances while a vehicle is stopped for a long period of time, thereby leaving a possibility for an over discharge.
Therefore, for a lead storage battery to be used in vehicles carrying these systems, service life characteristics under a usage mode in which charge/discharge is repeated frequently with a lower SOC range need to be improved.
For deterioration factors of a lead storage battery under such usage mode, an insufficient charge due to decline in chargeability of the lead storage battery can be mentioned, mainly. Since a charge system of a vehicle is based on constant voltage control, when chargeability of a negative electrode plate is reduced, a potential of negative electrode decreases at an initial stage of charge and a voltage rapidly rises up to a preset voltage value, and a current decreases sooner. Thus, a sufficient amount of charged electricity of a lead storage battery can not be secured, thereby rendering the battery to be in an undercharged state.
For suppressing such deterioration, there has been proposed a method in which a lead alloy layer containing Sn and Sb is formed on a surface of a positive electrode grid of a Pb—Ca—Sn alloy, for example (Patent Document 1). The formation of such layer will suppress a deterioration of the positive electrode active material and a formation of a passivated layer at an interface between the positive electrode active material and the positive electrode grid.
Also, a part of Sb which exists on a surface of the positive electrode grid dissolves in an electrolyte, and deposits on a negative electrode plate. The deposited Sb on a negative electrode active material raises a charging potential of the negative electrode plate, and a charging voltage lowers down, thereby improving chargeability of a lead storage battery. As a result, deterioration of the lead storage battery due to insufficient charge during charge/discharge cycle is suppressed.
This method is very effective in an engine starting lead storage battery which is used while SOC is over 90%, and service life characteristics will drastically improve.
However, when a lead storage battery is to be used in a vehicle equipped with the above stop-and-go-system or the regenerative-braking-system, that is, when a lead storage battery is to be used in a mode in which charge/discharge is repeated under lower SOC range, there was a problem in that the water content in the electrolyte rapidly decreases at the end of its service life, while the chargeability could be secured.
When the water content in the electrolyte decreases, a negative electrode strap and a tab of negative electrode grid are exposed from the electrolyte. By being exposed to the oxygen in the air, a welded part of the strap and the tab is corroded, leading to a possibility of a disconnection.
Also, even the negative electrode strap and the tab of negative electrode grid are being immersed in the electrolyte, the tab of negative electrode grid is easily being corroded when Sb is deposited in a very small amount on a surface of the tab of negative electrode grid by dissolution of Sb included in the positive electrode grid, and in a positive electrode connecting member comprising a positive electrode strap and a positive electrode pole or a positive electrode connecting body into the electrolyte.
Additionally, there has been proposed in Patent Document 2 to dispose a separator comprising a glass fiber mat having resistance to acids between the positive electrode plate and the negative electrode plate, in order to suppress a separation of the positive electrode active material. When the glass fiber mat is used, an internal pressure of a battery increases compared with the case when a micro-porous polyethylene sheet, which is commonly used in a starter lead storage battery, is used. Such increase in the internal pressure caused a problem: chargeability of the negative electrode plate declined, and a service life of the battery is shortened.
Also, although a lignin compound as an expander and carbon as a conductive material to be added to the negative electrode active material layer have effects to improve chargeability of the negative electrode plate, since these additives decompose or dissolve from the active material layer, it is difficult to maintain the effects. Also, when the amounts of these additives are increased to maintain the effects, there is a possibility to cause a problem in battery performances such as declines in chargeability and discharge characteristics at an initial period. There is also a possibility to cause a problem in manufacturing process: the negative electrode paste can not be charged to the negative electrode grid in good condition, and unevenness in the amount of the negative electrode paste to be retained increases.
Patent Document 1: Japanese Laid-Open Patent Publication No. Hei 3-37962
Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 7-94205